add function getargnum

add function getargnum

For cmdline argument with numeric value, add a new function getargnum
It will get proper value with default value as $1, min value as $2,
max value as $3, and param name as $4. valid result will be echo to stdout.
for nul or value not valid it will just echo the default value.
Note: The values should be >=0

[v1->v2]: add arg <minval>
[v2->v3]: do not use bash string match =~

Signed-off-by: Dave Young <dyoung@redhat.com>