issue#6: Add benchmark for opening device files

As we found out in <#6>,
opening the character device special files for the more modern
evdev API takes about 5000 times the time needed for opening the
device for the old console API:

    console API
        device name: /dev/ttyN (e.g. /dev/tty4)
        time per open-and-close cycle: about 3us

    evdev API
        device name: /dev/input/by-path/platform-pcspkr-event-spkr
        time per open-and-close cycle: about 15ms

To help with measuring those times, and timing them with
/usr/bin/time we have now added "issue-6-benchmark".

Run "make issue-6-benchmark" to build "issue-6-benchmark".

Read "issue-6-benchmark.c" or run "issue-6-benchmark --help"
for usage information.

One peculiar thing to note is that apparently, the evdev API
takes neither User time nor System time, but still needs wall
clock time:

	Command being timed: "./issue-6-benchmark 5118303 /dev/tty2"
	User time (seconds): 0.54
	System time (seconds): 13.37
	Percent of CPU this job got: 94%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 0:14.75

	Command being timed: "./issue-6-benchmark 958 /dev/input/by-path/platform-pcspkr-event-spkr"
	User time (seconds): 0.00
	System time (seconds): 0.07
	Percent of CPU this job got: 0%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 0:14.66

.gitignore

@@ -4,6 +4,7 @@
 /beep
 /*.clang
 /*.gcc
+/issue-6-benchmark
 
 # Built sources
 /beep-usage.c

GNUmakefile

@@ -153,6 +153,10 @@ beep_OBJS += beep-library.o
 beep_OBJS += beep-usage.o
 beep_OBJS += beep-drivers.o
 
+check_PROGRAMS        += issue-6-benchmark
+issue_6_benchmark_OBJS = issue-6-benchmark.o
+issue_6_benchmark_LIBS = -lm
+
 # The drivers here use `__attribute__((constructor))` functions to
 # register themselves with `beep_drivers_register()`, so the last one
 # listed here will be linked last and have its constructor called at