meson: drop "meson-" prefix from various helper script filenames

meson: drop "meson-" prefix from various helper script filenames

In a few cases, the prefix was originally necessary because a different helper
script was used for automake, and a different one for meson. But now we use
meson exclusively, and the prefix isn't useful. This also synchronizes the
target name, file name, and variable name in meson.build. The targets exposed
by meson didn't have the prefix, so the user interface is unchanged.

(The prefix is retained in the few tools that are used for meson itself,
e.g. meosn-vcs-tag.sh, meson-make-symlink.sh, etc.)