fix getopt_long_only misinterpreting "--" as an option

simplify part of getopt_long

as a result of commit e8e4e56a8ce1f3d7e4a027ff5478f2f8ea70c46b,
the later code path for setting optarg to a null pointer is no longer
necessary, and removing it eliminates an indention level and arguably
makes the code more readable.

always set optarg in getopt_long

the standard getopt does not touch optarg unless processing an option
with an argument. however, programs using the GNU getopt API, which we
attempt to provide in getopt_long, expect optarg to be a null pointer
after processing an option without an argument.

before argument permutation support was added, such programs typically
detected its absence and used their own replacement getopt_long,
masking the discrepency in behavior.

fix regression in getopt_long support for non-option arguments

commit b72cd07f176b876aa51864d93aa8101477b1d732 added support for a
this feature in getopt, but it was later broken in the case where
getopt_long is used as a side effect of the changes made in commit
91184c4f16b143107fa9935edebe5d2b20bd70d8, which prevented the
underlying getopt call from seeing the leading '-' or '+' character in
optstring.

this commit changes the logic in the getopt_long core to check for a
leading colon, possibly after the leading '-' or '+', without
depending on the latter having been skipped by the caller. a minor
incorrectness in the return value for one error condition in
getopt_long is also fixed when opterr has been set to zero but
optstring has no leading ':'.

set optopt in getopt_long

this is undocumented but possibly expected behavior of GNU
getopt_long, and useful when error message printing has been
suppressed.

add error message printing to getopt_long and make related improvements

some related changes are also made to getopt, and the return value of
getopt_long in the case of missing arguments is fixed.

simplify getopt_long argv permutation loop logic

fix handling of "--" with getopt_long argv permutation

if argv permutation is used, the option terminator "--" should be
moved before any skipped non-option arguments rather than being left
in the argv tail where the caller will see and interpret it.

accept null longopts pointer in getopt_long

this is an undocumented feature of GNU getopt_long that the BSD
version also mimics, and is reportedly needed by some programs.

support abbreviated options in getopt_long

support options after non-option arguments in getopt_long (argv permutation)

add support for non-option arguments extension to getopt

this is a GNU extension, activated by including '-' as the first
character of the options string, whereby non-option arguments are
processed as if they were arguments to an option character '\1' rather
than ending option processing.

add getopt reset support

based on proposed patches by Daniel Cegiełka, with minor changes:
- use a weak symbol for optreset so it doesn't clash with namespace
- also reset optpos (position in multi-option arg like -lR)
- also make getopt_long support reset

cleanup src/linux and src/misc trees, etc.

previously, it was pretty much random which one of these trees a given
function appeared in. they have now been organized into:

src/linux: non-POSIX linux syscalls (possibly shard with other nixen)
src/legacy: various obsolete/legacy functions, mostly wrappers
src/misc: still mostly uncategorized; some misc POSIX, some nonstd
src/crypt: crypt hash functions

further cleanup will be done later.