fix shifts possibly larger than type in major() macro

in theory this should not be an issue, since major() should only be
applied to type dev_t, which is 64-bit. however, it appears some
applications are not using dev_t but a smaller integer type (which
works on Linux because the kernel's dev_t is really only 32-bit). to
avoid the undefined behavior, do it as two shifts.

support full range of dev_t major/minor numbers in makedev, etc. macros

sysmacros major/minor: result should have type unsigned int, not dev_t

header file fixes: multiple include guard consistency and correctness

one file was reusing another file's macro name, and many had
inconsistent underscores and application of SYS prefix, etc.

patch by Szabolcs Nagy (nsz)

cleanup more bits cruft (sysmacros and socket)

initial check-in, version 0.5.0