refactor group file access code

this allows getgrnam and getgrgid to share code with the _r versions
in preparation for alternate backend support.

include cleanups: remove unused headers and add feature test macros

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.