Squashed commit of the following:
Last minute clean-up
libs/libc/grp/lib_find_grpfile.c: Fix some problems found in testing.
libs/libc/pwd: Finishes off implementation using /etc/passwd.
libs/libc/grp: Finishes off implementation using /etc/group.
libs/libc/pwd/lib_find_pwdfile.c: Add logic to look up user information in /etc/passwd file, if available.
libs/libc/grp/lib_find_grpfile.c: Add logic to look up group information in /etc/group file, if available.
Added stubbed implementations of getpwuid, getpwuid_r, getpwnam, and
getpwnam_r. These functions are essentially stubs pretending that NuttX
supported users and that the only configured user in the system was 'root'
with uid 0. The intend is not to provide meaningful new features, but to ease
porting of foreign source code to NuttX.
