MINOR: config: make MAX_PROCS configurable at build time

MINOR: config: make MAX_PROCS configurable at build time

For some embedded systems, it's pointless to have 32- or even 64- large
arrays of processes when it's known that much fewer processes will be
used in the worst case. Let's introduce this MAX_PROCS define which
contains the highest number of processes allowed to run at once. It
still defaults to LONGBITS but may be lowered.