automatically select argument to `remote add` as current remote

[preminger]

When adding a remote via singularity remote add <remote_name> <remote_uri>, the newly-added remote should be selected as the current remote by default.

A flag (--notcurrent) should be added to suppress this behavior if needed.

ETA: The flag should be --not-current

Discussion: #1639

[dtrudg]

Note that it should be --not-current or similar... we have been attempting to move toward consistent - for word separation in flags, e.g.

$ singularity run --nohttps docker://alpine
Flag --nohttps has been deprecated, use --no-https