processes: use long for pids on linux and solaris

Up to now, we had a mix of `int`, `long` and `pid_t` to represent the
variable part of `/proc/<PID>/status` and friends. This patch
standardizes on `long` on the Linux and Solaris platforms.

The max size of `int` is smaller than the maximum number of processes
current kernels support. `pid_t` is used to ensure portability of
functions such as `getpid()`, which aren't used in this plugin, and
apparently resolves to an `int` on Solaris.

This is a follow up to bb978c1, which triggered a format string issue on
solaris 64 bit.