xseg_posixfd: Add S_ISGID bit to fifo's directory

Add S_ISGID bit to fifo's directory. This way each fifo created under this
directory inherits the group id of the POSIXFD directory. This enables different
Archipelago components to signal each other independently from their effective
user id.
xseg: Do not create posixfd directory

Do not implicitly create posixfd directory. Instead, expect it to be already
setup correctly and return an error otherwise.
xlock: Replace label based PC with asm based

Replace the possible error-prone way of getting the current program counter from
labels. Since the previous implementations could be greatly influnced by the
compiler's behaviour, switch to an assembly based approach.