util: continuation support for load_env_file

Variable definitions can be written on more than one line - if each ends
with a backslash, then is concatenated with a previous one. Only
backslash and unix end of line (\n) are treated as a continuation.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=58083

[zj: squashed two patches together; cleaned up grammar; removed
     comment about ignoring trailing backslash -- it is not ignored.]

Document continuation support in systemd.exec
util: modernization and test for load_env_file