optimize get_current_dir_name to reduce stack bloat

our getcwd already (as an extension) supports allocation of a buffer
when the buffer argument is a null pointer, so there's no need to
duplicate the allocation logic in this wrapper function. duplicating
it is actually harmful in that it doubles the stack usage from
PATH_MAX to 2*PATH_MAX.

include cleanups: remove unused headers and add feature test macros

fix get_current_dir_name behavior

add get_current_dir_name function