Fix typos found by misspellings

Fix typos found by misspellings

The tool misspellings (https://github.com/lyda/misspell-check)
detected several typos. Command used:

  $ git ls-files | grep -v ^po/ | misspellings -f -

* isosize: Fix typo in usage string.
* configure.ac: Fix typo in help string of --enable-most-builds option.
* fdisk: Fix typo in man page.
* libblkid, blkid, mount: Likewise.
* Fix various typos in docs and in source code comments.

Signed-off-by: Bernhard Voelker <mail@bernhard-voelker.de>