[manpages] Rewrite man pages in a simpler text format.

Makefile still needs work, but I wanted to commit to save progress.

man8/era_invalidate.txt

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+NAME
+  era_invalidate - Provide a list of blocks that have changed since a particular era.
+
+SYNOPSIS
+  era_invalidate [options] {device|file}
+
+DESCRIPTION
+  era_invalidate examines era metadata and lists blocks that may have changed
+  since a given era.
+
+  This tool cannot be run on live metadata unless the --metadata-snap option is
+  used.
+
+OPTIONS
+  -h, --help	Print help and exit.
+  -V, --version	Print version information and exit.
+  -o {output file}	Write output to a file rather than stdout.
+  --metadata-snapshot	Use the metadata snapshot rather than the current superblock.
+  --written-since	Blocks written since the given era will be listed.
+
+EXAMPLE
+  List the blocks that may have been written since the beginning of era 13 on the
+  metadata device /dev/vg/metadata:
+
+    $ era_invalidate --written-since 13 /dev/vg/metadata
+
+  The device may not be actively used by the target when running.
+
+DIAGNOSTICS
+  era_invalidate returns an exit code of 0 for success or 1 for error (eg,
+  metadata corruption).
+
+SEE ALSO
+  era_check(8), era_dump(8), era_repair(8), era_restore(8)
+
+AUTHOR
+  Joe Thornber <ejt@redhat.com>