snap-seccomp: use username regex as defined in osutil/user.go
snapd should standardize on a single username(/groupname) regex. While
today the osutil/user.go regex is used for adding users to the system,
snap-seccomp uses a more restricted regex. Since some day we plan to
support snapd adding system, shared and private users, as well as
adjusting ACLs/udev rules for device access for users (eg, such as ones
created by snapd in the first place on Ubuntu Core), it makes sense to
standardize on one regex to avoid users being created that can't be
referenced in other parts of snapd.
The old regex, `^[a-z0-9][-a-z0-9+.-_]*$`, had a bug in it where it
would accept any character within the range of '.-_' where 'a:b'
would match since '.' < ':' < '_' in the ASCII table.
Make IsValidUsername public since it makes adding tests easier and
subsequent commits will use this.
