[pmemchk] Test Bash version >=4.0.0

Some of the code requires Bash version 4.0 or later to work, so a check needs to be added to exit if the user has an old version.

if ((BASH_VERSINFO[0] < 4))
then
  echo "Sorry, you need at least bash-4.0 to run this script."
  exit 1
fi

Here's a breakdown of the BASH_VERSINFO array:

# Bash version information:

for n in 0 1 2 3 4 5
do
  echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}"
done

# BASH_VERSINFO[0] = 5                        # Major version no.
# BASH_VERSINFO[1] = 1                        # Minor version no.
# BASH_VERSINFO[2] = 8                        # Patch level.
# BASH_VERSINFO[3] = 1                        # Build version.
# BASH_VERSINFO[4] = release                  # Release status.
# BASH_VERSINFO[5] = x86_64-redhat-linux-gnu  # Architecture
                                              # (same as $MACHTYPE).

# bash --version
GNU bash, version 5.1.8(1)-release (x86_64-redhat-linux-gnu)
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

sscargal / pmemchk

[pmemchk] Test Bash version >=4.0.0 #124