fix various header namespace issues under feature-test-macro control

reported and changes suggested by Daniel Sabogal.

remove parameter names from public headers

inclusion of these names was unintentional and in most cases is a
namespace violation. Daniel Sabogal tracked down and reported these.

make brace placement in public header struct definitions consistent

placing the opening brace on the same line as the struct keyword/tag
is the style I prefer and seems to be the prevailing practice in more
recent additions.

these changes were generated by the command:

find include/ arch/*/bits -name '*.h' \
-exec sed -i '/^struct [^;{]*$/{N;s/\n/ /;}' {} +

and subsequently checked by hand to ensure that the regex did not pick
up any false positives.

add put*ent functions for passwd/group files and similar for shadow

since shadow does not yet support enumeration (getspent), the
corresponding FILE-based get and put versions are also subbed out for
now. this is partly out of laziness and partly because it's not clear
how they should work in the presence of TCB shadow files. the stubs
should make it possible to compile some software that expects them to
exist, but such software still may not work properly.

fix missing type error in grp.h from adding fgetgrent

add fgetgrent function

based on patch by Isaac Dunham, moved to its own file to avoid
increasing bss on static linked programs not using this nonstandard
function but using the standard getgrent function, and vice versa.

default features: make musl usable without feature test macros

the old behavior of exposing nothing except plain ISO C can be
obtained by defining __STRICT_ANSI__ or using a compiler option (such
as -std=c99) that predefines it. the new default featureset is POSIX
with XSI plus _BSD_SOURCE. any explicit feature test macros will
inhibit the default.

installation docs have also been updated to reflect this change.

support _BSD_SOURCE feature test macro

patch by Isaac Dunham. matched closely (maybe not exact) to glibc's
idea of what _BSD_SOURCE should make visible.

fix prototypes/signature for setgroups, etc.

initial check-in, version 0.5.0