v0.2-b

Changes from v0.2-a:

• Updated README.txt;
• Wrote realpath() at lib/posix-alt.shi, also made it glob file names;
• Completely rewrote nproc() at lib/posix-alt.shi;
• Treat $MITZUNE_PREFIX with realpath();
• Wrote functions that import/export prefixes;
• Portabilize check_doas()/elevate() between @slicer69's doas versions;
• Made decChrootFunction read-only, for safety reasons;
• Portabilize between UNIX tools implementations.