For code reuse in Linuxulator rename get_proccess_cputime()
and get_thread_cputime() and add prototypes for it to <sys/syscallsubr.h>.

As both functions become a public interface add process lock assert
to ensure that the process is not exiting under it.

Fix whitespace nit while here.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D23340
MFC after 2 weeks