bash-completion: avoid losing backslashes in unit names

bash-completion: avoid losing backslashes in unit names

Use 'read -r' everywhere to consider backslashes as parts of the input line.
Single-quote the arguments to 'compgen -W' to avoid immediate expansion.
compgen itself will expand the argument.

Fixes a possible reason for "Failed to issue method call: Unknown unit"
after requesting completion.

https://bugzilla.redhat.com/show_bug.cgi?id=814966