[kernel/libc] Allow for concurrent operation of libc and kernel precision timer

[ghaerr]

The 8254 PIT countdown register is changed to be read during the interrupts disabled critical section, which allows for concurrent use of the precision timer get_ptime function from both the kernel and an application process performing measurements at the same time. This may be useful when measuring the ktcp main loop and also looking at a NIC driver packet read/write response time, for instance.

Discussed briefly in #1963 and Mellvik/TLVC#71.

Moves ptostr routine into libc/misc with the other numeric to string conversion routines.
Use write rather than printf for errors in init_ptime() to keep dependencies down.