Rework signal code to allow using it by other modules, like linprocfs:

1. Linux sigset always 64 bit on all platforms. In order to move Linux

sigset code to the linux_common module define it as 64 bit int. Move
Linux sigset manipulation routines to the MI path.

2. Move Linux signal number definitions to the MI path. In general, they

are the same on all platforms except for a few signals.

3. Map Linux RT signals to the FreeBSD RT signals and hide signal conversion

tables to avoid conversion errors.

4. Emulate Linux SIGPWR signal via FreeBSD SIGRTMIN signal which is outside

of allowed on Linux signal numbers.

PR: 197216