fix type of extended argument array to pselect6 syscall

this only matters on x32 (and perhaps future 32-on-64 abis for other
archs); otherwise the type is long anyway. the cast through uintptr_t
prevents nonsensical "sign extension" of pointers, and follows the
principle that uintptr_t is the canonical integer type to which
pointer conversion is safe.
fix placement of multiple inclusion guard endif in internal syscall.h

this was messed up during a recent commit when the socketcall macros
were moved to the common internal/syscall.h, and the following commit
expanded the problem by adding more new content outside the guard.
fix sys_open macro for archs without the plain open syscall
overhaul tmpfile, tmpnam, and tempnam functions

these all now use the shared __randname function internally, rather
than duplicating logic for producing a random name. incorrect usage of
the access syscall (which works with real uid/gid, not effective) has
been removed, along with unnecessary heavy dependencies like snprintf.
fix missing declaration of strcpy in implementation of tmpnam