Fix shell globbing bugs

There are several problems leading to the unintentional globbing issue:

Firstly, within `rcup` and `rcdn`, when constructing arguments to pass
to `lsrc`, the _for_ loops over the arguments do not have quoted
variables, leading to globbing.  I have quoted these accordingly.

Secondly, `lsrc` is invoked as follows:

```sh
dests_and_srcs="$(lsrc $LS_ARGS)"
```

When shells use command substitution like this, they go through two stages:

- Word expansion. This is useful because it splits `LS_ARGS` back up
into its constituent strings.
- File name expansion. The side effect of this is to introduce globbing.

You can read more about how this works [here](https://www.tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_04.html#sect_03_04_07).

To fix this, I have passed `lsrc` and its arguments to `eval`.  This
involves quoting the relevant arguments, so:

```sh
for dotfiles_dir in "$DOTFILES_DIRS"; do
  LS_ARGS="$LS_ARGS -d $dotfiles_dir"
done
```

becomes

```sh
for dotfiles_dir in "$DOTFILES_DIRS"; do
  LS_ARGS="$LS_ARGS -d \"$dotfiles_dir\""
done
```

Then `lsrc` is invoked as follows:

```sh
dests_and_srcs="$(eval "lsrc $LS_ARGS")"
```

There is one final non-globbing issue: the parsing of arguments can
introduce extra spaces in the variables, which then trip up the
`dotfiles_dir_excludes` function. For example:

```sh
I) includes="$includes $OPTARG";;
```

introduces a space if `includes` is empty or null.

I have introduced the function `append_variable`, which allows two
variables to be appended without introducing unnecessary whitespace.
Then the additional whitespace is never added in the first place.

Fixes #256.

1 files changed, 8 insertions, 0 deletions

diff --git a/share/rcm.sh.in b/share/rcm.sh.in
index f7c6137..5d0a373 100644
--- a/share/rcm.sh.in
+++ b/share/rcm.sh.in
@@ -171,6 +171,14 @@ decode() {
   echo "$file" | tr "$DELIMITER" " "
 }
 
+append_variable() {
+  if [ -z "$1" ]; then
+    echo "$2"
+  else
+    echo "$1 $2"
+  fi
+}
+
 : ${RCRC:=$HOME/.rcrc}
 
 if [ -r "$RCRC" ]; then