Fix bugs in and expand syscall emulation code.

Fix bugs in and expand syscall emulation code.

arch/alpha/fake_syscall.cc:
    Fix a couple of bugs:
    - error return codes weren't making it through due to inadvertent cast to unsigned
    - sigreturn broken in not one but two ways
    - make all file descriptors look like plain files (not ttys)
    Added implementations of setuid, getgid, fcntl, and getdirentries from Dave Oehmke

--HG--
extra : convert_revision : 53d3f13e1b05e3bde9e68ada3774ca39fa4c0d4c