Remove hard coded "Linux" OS from manpages

The recommended practice for `.Os` on FreeBSD is to not specify any
arguments.  The correct OS name is used automatically.

Oddly enough, on the Linux distro I tested this on (CentOS 7), the man
pager defaulted to displaying "BSD" as the OS rather than "Linux".  To
accommodate this, tack " Linux" back on in an install hook on Linux.
This is much simpler than removing it for FreeBSD when vendored in the
base system.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10760

man/man1/Makefile.am

@@ -1,2 +1,12 @@
 dist_man_MANS = zhack.1 ztest.1 raidz_test.1 zvol_wait.1 arcstat.1
 EXTRA_DIST = cstyle.1
+
+if BUILD_LINUX
+# The man pager in most Linux distros defaults to BSD instead of Linux
+# when .Os is blank, but leaving it blank makes things a lot easier on
+# FreeBSD when OpenZFS is vendored in the base system.
+install-data-hook:
+	cd $(DESTDIR)$(mandir)/man1; \
+	$(SED) ${ac_inplace} -e 's/^\.Os$$/.Os Linux/' \
+	    $(dist_man_MANS)
+endif

man/man1/zvol_wait.1

@@ -1,6 +1,6 @@
 .Dd July 5, 2019
 .Dt ZVOL_WAIT 1 SMM
-.Os Linux
+.Os
 .Sh NAME
 .Nm zvol_wait
 .Nd Wait for ZFS volume links in