small fixes to libuv locks

[JeffBezanson]

• use normal locking instead of nogc
• remove locks before threads have started
• add a trylock function (previously we only had trylock_nogc)

[vtjnash]

  Error while loading expression starting at /tmp/julia/share/julia/test/cmdlineargs.jl:87
  caused by [exception 1]
  failed process: Process(`/tmp/julia/bin/julia -Cnative -J/tmp/julia/lib/julia/sys.so --depwarn=error --check-bounds=yes -g1 --startup-file=no -v`, ProcessSignaled(11)) [0]

[JeffBezanson]

Looks to me like jl_parse_opts should not call jl_exit, since julia_init has not been called yet.