added .float spawnshieldtime which is used to grant temporary

added .float spawnshieldtime which is used to grant temporary
invulnerability to newly spawned players (if g_spawnshieldtime is not
0), and also used internally by g_midair mode (along with
g_midair_shieldtime cvar), this means that
players are invulnerable for a short time after leaving the floor in
midair mode, so that multishot weapons (crylink, shotgun, electro
secondary) don't do damage on the second and later hits in a single
frame (because their first hit cleared the onground flag)
added g_spawnshieldtime cvar
added g_midair_shieldtime cvar

git-svn-id: svn://svn.icculus.org/nexuiz/trunk@1075 f962a42d-fe04-0410-a3ab-8c8b0445ebaa