Add some CLOEXEC constants

Add support to build as part of the standard library

Touch up some SA_* constants

Use bindings for -D_FILE_OFFSET_BITS=64

They're all added under the `foo64` names with the `foo64` types added as well,
so they still need to be explicitly chosen.

Add more bindings for signals and select/pselect

Define dirent_t on all platforms

Be more selective about skipped pthread on musl tests

Fixup the value of PTHREAD_STACK_MIN as well

Implement RFC 1291 changes

Conflicts:
	rust