Throughout, change fdtab references to cygheap->fdtab.

Throughout, change fdtab references to cygheap->fdtab.
* child_info.h (cygheap_exec_info): Eliminate special fdtab stuff.
* spawn.cc (spawn_guts): Ditto.
* cygheap.cc (cygheap_init): Initialize fdtab, if appropriate.
* cygheap.h (CYGHEAPSIZE): Include size of init_cygheap.
(_cmalloc_entry): Include fdtab here.
* dtable.h (dtable): Declare/define new methods.
* dtable.cc (dtable::vfork_child_fixup): New method.
(dtable::fixup_after_exec): Remove unneeded extra arguments.
* dcrt0.cc (dll_crt0_1): Ditto.
* environ.cc (getwinenv): Use case sensitive comparison.
(winenv): Make a copy of environment cache to avoid realloc problems when
duplicate environment variables exist in the environment.  (From Egor Duda)
* net.cc (cygwin_socket): Revert Apr 14 change.
* include/sys/file.h: Protect against previous X_OK definition.
* passwd.cc: Eliminate passwd_sem throughout.
* security.cc: Ditto.
* cygwin.din: Export New functions.
* passwd.cc (read_etc_passwd): Make race safe.
(getpwuid_r): New function.
(getpwnam_r): New function.