ssh/knownhosts: treat only ASCII space and tab as whitespace

The previous implementation used bytes.TrimSpace, which strips all
Unicode whitespace categories (e.g., non-breaking spaces). However,
OpenSSH's known_hosts parser (hostfile.c) strictly treats only ASCII
space (0x20) and horizontal tab (0x09) as separators.

This discrepancy meant the Go parser might interpret fields differently
than OpenSSH, potentially treating parts of a key or hostname as
separators if they contained Unicode whitespace.

This change replaces bytes.TrimSpace with a local trimSpace helper that
only trims " \t", ensuring parsing behavior consistent with the
reference implementation.

This issue was found during a security audit by NCC Group Cryptography
Services, sponsored by Teleport, and was assessed and is being fixed as
a non-security bug.

Change-Id: Ia536889636de2c167d2507c01e3f1b7c033c9a8f
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/782426
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>