devproc: fix syscalltrace error handling, conistent use of nil for pointers
devproc: nil
page: fix deadlock with addpage (thanks BurnZeZ for the stacktraces)

addpage() should not be called with the display locked as it
calls showpage1() which sleeps when there are too many
processes active.

the bug was triggered by plumbing to trigger the addpage().
libframe: use correct text color on frinsert depending on selection (thanks sl)

frinsert() used cols[TEXT] for drawing selected text instead of the
HTEXT color. this was discovered with rio -b.
a.out(6): sync definitions
pc64: preserve user extern registers R14 and R15 across syscalls, use Ureg.bp (RARG) for syscall number

the 6c compiler reserves R14 and R15 for extern register variables,
which is used by the kernel to hold the m and up pointers. until
now, the meaning of R14 and R15 was undefined for userspace and
extern register would not work as the kernel trashes R14 and R15
on syscalls. with this change, user extern registers R14 and R15
are zeroed on exec and otherwise preserved across syscalls. so
userspace *could* use them for per process variables like the
kernel does.

use Ureg.bp (RARG) for syscall number instead of Ureg.ax. this is
less confusing and mirrors the amd64 calling convention.