cmd: rename *.C to *.c

These were *.C back in 2003, before the 9 script, to avoid conflicts
with the system utilities. A later change renamed them, but that
change seems to have been lost during the hg->git conversion
because I ran the conversion on a case-insensitive file system.

Change-Id: Id32c99cb9571ef0e185c3cc9e8c8d6d5b48ca195

src/cmd/du.c

diff --git a/src/cmd/du.C b/src/cmd/du.c
similarity index 100%
rename from src/cmd/du.C
rename to src/cmd/du.c

src/cmd/ls.c

diff --git a/src/cmd/ls.C b/src/cmd/ls.c
similarity index 100%
rename from src/cmd/ls.C
rename to src/cmd/ls.c

src/cmd/md5sum.c

diff --git a/src/cmd/md5sum.C b/src/cmd/md5sum.c
similarity index 100%
rename from src/cmd/md5sum.C
rename to src/cmd/md5sum.c

src/cmd/mkdir.c

diff --git a/src/cmd/mkdir.C b/src/cmd/mkdir.c
similarity index 100%
rename from src/cmd/mkdir.C
rename to src/cmd/mkdir.c