cfdisk: data type mismatch, and other, compiler warning fixes

cfdisk: data type mismatch, and other, compiler warning fixes

Following warnings will longer appear when one will compile with
gcc flags -Wall -Wextra -pedantic

cfdisk.c:475:3: warning: comparison of unsigned expression < 0 is always false
cfdisk.c:487:16: warning: comparison between signed and unsigned integer expressions
cfdisk.c:492:14: warning: comparison between signed and unsigned integer expressions
cfdisk.c:565:19: warning: comparison between signed and unsigned integer expressions
cfdisk.c:569:19: warning: comparison between signed and unsigned integer expressions
cfdisk.c:1070:14: warning: comparison between signed and unsigned integer expressions
cfdisk.c:1568:5: warning: missing initializer
cfdisk.c:1568:5: warning: (near initialization for 'tmp_ext.volume_label')

mbsalign.c:131:2: warning: comparison of unsigned expression >= 0 is always true

Signed-off-by: Sami Kerola <kerolasa@iki.fi>