Cherry-pick 7.10 changes to 8.0.1.

Cherry-picked commits:
    e24220ea gdb: disable python when python is not available.
    d73c5467 use snprintf instead of strlcpy for portablitly
    c64829b2 Fallback to open if multifs_open fails.
    21770732 Fix gdbserver process interruption.
    b711aa82 Add Unix domain socket support to gdbserver 7.9.1

Test: build.py passes with gdb 8.0.1.