man: explicitly say when multiple units can be specified

itistoday> how do you specify multiple dependencies in a unit file? i've been
           googling and can't find this basic thing :-\
itistoday> do you use a comma, or use multiple After= statements?
systemctl: refactor show()

v2, don't leave in unused function
systemctl: add "systemctl cat"
systemctl: do not show SourcePath when FragmentPath cannot be found

Those files can be in a completely deferent format and also
arbitrarily long, and usually contain information about other
stuff. If we ever add SourceLine= or SourceLines= in addition
to SourcePath=, and can show the relevant information only, this
commit can be reverted.
shell-completion: systemctl cat

Also update -H help string to follow the binaries.
fix regression of read_full_file() in fileio.c

my e93c33d4aa broke this stupidly