Build the other example filesystems with CC/CXX.
Use $MAKE and make -C for recursive targets.
Set libprocfs_sequencegrab install name to /usr/local/lib

procfs assumes a prefix of /usr/local elsewhere, although this isn't actually installed.
-Wall fixes: signedness
-Wall fixes: don't assume string literals are writable.
-Wall fixes: syntax and format mismatches
procfs: Don't overwrite the end of argv.

new_argv was already being allocated and filled presumably for this purpose.
Fix potential junk reads.
Plug procfs leaks.
unixfs: drop dead code.

Most being shared bits from sibling filesystems which became unused.
unixfs: Bring remaining dead stores back to life as error handlers.