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

man/systemd.exec.xml

@@ -299,9 +299,11 @@
                                 contain new-line separated variable
                                 assignments. Empty lines and lines
                                 starting with ; or # will be ignored,
-                                which may be used for commenting. The
-                                parser strips leading and
-                                trailing whitespace from the values
+                                which may be used for commenting. A line
+                                ending with a backslash will be concatenated
+                                with the following one, allowing multiline variable
+                                definitions. The parser strips leading
+                                and trailing whitespace from the values
                                 of assignments, unless you use
                                 double quotes (").</para>