udev-util: bound leading whitespace skip in udev_replace_whitespace (#42757)

udev_replace_whitespace() is documented to read at most 'len' bytes from
'str':
- the strspn() skip of leading whitespace stops at a non-space byte or
NUL, not at 'len'
- ata_id passes the space padded, non-NUL-terminated ATA IDENTIFY
model/serial/fw fields
- an all-blank field reads off the end of the 512-byte hd_driveid stack
struct
Capped the skip to 'len'; existing outputs are unchanged. Added a
regression test.

diff --cc src/basic/string-util.c
Simple merge

diff --cc src/basic/string-util.h
Simple merge

diff --cc src/test/test-string-util.c
Simple merge